German Published Patent Application Number 195 37 814 describes the structure of a layer system and a method for hermetically capping sensors, in particular surface-mounted micromechanical rotational speed sensors or acceleration sensors. The manufacture of the sensor structure using conventional technological methods is explained. The sensor is capped using a separate cap wafer made of silicon that is structured using complex structuring processes such as KOH etching. The cap wafer is placed on the sensor wafer at the wafer level, using a glass solder (seal glass). For this purpose a wide bond frame is needed around each sensor chip, in order to ensure adequate adhesion and sealing of the cap wafer.
An anodic bond between a cap wafer and a sensor wafer is conceivable, where electrical contacts to the sensor wafer from the rear surface of a Si substrate are created by isolation trenches.
One problem with such an anodic capping method is that the glass silicon composite wafer used (preferably Pyrex glass) is expensive to manufacture. The sequence of grinding, polishing and etching processes increases the process costs. Process reliability, in particular for adjusting the thickness of the glass wafer, is only attainable by specifying broad tolerance limits. Structuring the cap cavity in Pyrex using wet chemical methods such as BOE (buffered oxide etch) is expensive, and in the case of great etching depths of, for example, 5-10 μm, runs into problems in regard to varnish adhesion and surface roughness.
A significant disadvantage of the technical design appears in regard to the anodic bonding with simultaneous formation of a metal-semiconductor contact. The incorporated metal inlay which is intended to keep the cavity space field-free during anodic bonding overlaps with the bond frame, introducing a rigid spacer which severely interferes with the bond and may result in leakage.